1. Field of the Invention
The invention relates to fluid-operated brake actuators for vehicles and more particularly to a fluid-operated brake actuator with a bearing for a permanently installed caging tool.
2. State of the Prior Art
An air brake system for a vehicle such as a bus, truck or the like typically includes a brake shoe and drum assembly which is actuated by means of an actuator assembly operated by the selective application of a fluid such as compressed air. Conventional air brake actuators have both a service brake actuator for actuating the brakes under normal driving conditions by the application of compressed air and an emergency or spring brake actuator for actuating the brakes when the compressed air is released. The emergency brake actuator includes a strong compression spring which forces application of the brake when air is released. This actuator is often referred to as the spring brake. Typically, the spring brake actuator is disposed in tandem with the service brake actuator.
The service brake actuator is typically divided into two chambers by a diaphragm. Depressing the brake pedal during normal driving operation introduces compressed air into one of the chambers of the service brake actuator which, acting against the diaphragm, causes a service brake push rod in the opposite chamber to be extended and the brakes to be applied with an application force proportional to the air pressure in the service brake actuator.
In tandem actuator assemblies, a spring brake push rod typically extends from a chamber in the spring brake portion, through an aperture in a wall separating the spring brake actuator from the service brake actuator, and into a chamber in the service brake portion. Because at least one of the adjoining chambers is usually pressurized, a seal is provided at the aperture around the push rod comprising one or more O-rings positioned in annular channels in the wall around the aperture.
The spring brake actuator is typically divided into two chambers, a lower chamber and an upper chamber, separated by a rubber diaphragm and pressure plate, with the spring in the upper chamber acting between an end wall of the spring brake housing and a pressure plate. When full pressure is applied to the lower chamber, air pressure acting against the diaphragm and pressure plate compresses the spring, thereby retracting the pressure plate and diaphragm. The spring brake actuator rod is typically held in a retracted position by a relatively small return spring. In some actuators, the spring brake actuator rod is integral with the pressure plate and held in a retracted position by the air pressure. In both designs, the spring brake actuator rod does not affect the normal operation of the brake.
In the event of a loss of air pressure or an intentional exhaustion of air from the spring brake actuator, the brake will be mechanically activated by the force of the strong compression spring acting on the spring brake actuator rod which, in turn, acts upon the service brake push rod to apply the brakes. Thus, the spring brake portion serves both as a parking brake and an emergency brake.
A caging bolt or tool is used to cage the strong compression spring when, for example, the brake must be manually released. It is known to permanently install a caging bolt in a brake actuator. In one such actuator, a threaded bolt extends through a threaded opening in the actuator housing and into a hollow interior of the spring brake actuator rod, which is affixed to the pressure plate. As the emergency brake or parking brake is applied, the pressure plate and actuator rod reciprocate relative to the bolt. This reciprocation over time causes the threads of the bolt to wear.